U.S. patent number 8,637,334 [Application Number 12/938,948] was granted by the patent office on 2014-01-28 for high brightness light emitting diode covered by zinc oxide layers on multiple surfaces grown in low temperature aqueous solution.
This patent grant is currently assigned to The Regents of the University of California. The grantee listed for this patent is Steven P. DenBaars, Jun Seok Ha, Ingrid Koslow, Maryann E. Lange, Shuji Nakamura, Jacob J. Richardson, Daniel B. Thompson. Invention is credited to Steven P. DenBaars, Jun Seok Ha, Ingrid Koslow, Frederick F. Lange, Shuji Nakamura, Jacob J. Richardson, Daniel B. Thompson.
United States Patent |
8,637,334 |
Thompson , et al. |
January 28, 2014 |
High brightness light emitting diode covered by zinc oxide layers
on multiple surfaces grown in low temperature aqueous solution
Abstract
A high brightness III-Nitride based Light Emitting Diode (LED),
comprising multiple surfaces covered by Zinc Oxide (ZnO) layers,
wherein the ZnO layers are grown in a low temperature aqueous
solution and each have a (0001) c-orientation and a top surface
that is a (0001) plane.
Inventors: |
Thompson; Daniel B. (Walnut
Creek, CA), Richardson; Jacob J. (Santa Barbara, CA),
Koslow; Ingrid (Santa Barbara, CA), Ha; Jun Seok
(Goleta, CA), Lange; Frederick F. (Santa Barbara, CA),
DenBaars; Steven P. (Goleta, CA), Nakamura; Shuji (Santa
Barbara, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Thompson; Daniel B.
Richardson; Jacob J.
Koslow; Ingrid
Ha; Jun Seok
DenBaars; Steven P.
Nakamura; Shuji
Lange; Maryann E. |
Walnut Creek
Santa Barbara
Santa Barbara
Goleta
Goleta
Santa Barbara
Santa Barbara |
CA
CA
CA
CA
CA
CA
CA |
US
US
US
US
US
US
US |
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|
Assignee: |
The Regents of the University of
California (Oakland, CA)
|
Family
ID: |
43924450 |
Appl.
No.: |
12/938,948 |
Filed: |
November 3, 2010 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20110266551 A1 |
Nov 3, 2011 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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61257812 |
Nov 3, 2009 |
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61257811 |
Nov 3, 2009 |
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61257814 |
Nov 3, 2009 |
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Current U.S.
Class: |
438/29; 257/183;
257/E33.074; 438/46; 257/103; 257/E33.025; 257/98 |
Current CPC
Class: |
H01L
33/28 (20130101); H01L 33/32 (20130101); H01L
33/44 (20130101); H01L 33/14 (20130101); H01L
21/288 (20130101); H01L 33/007 (20130101); H01L
33/40 (20130101) |
Current International
Class: |
H01L
33/00 (20100101) |
Field of
Search: |
;257/183,E33.028,E33.067,98,E33.025 ;438/29,46 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
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90.degree. C.," Adv. Funct. Mater. 2006, 16, 799-804. cited by
applicant .
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to p-GaN and its application to high-brightness GaN-based
light-emitting diodes," Solid-State Electronics 49 (2005)
1381-1386. cited by applicant .
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oxide texturing window layers using natural lithography," Applied
Physics Letters 86, 221101 (2005). cited by applicant .
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GaN-buffered Al.sub.2O.sub.3 (0001) substrates by low-temperature
hydrothermal synthesis at 90.degree. C.," Adv. Funct. Mater. 2007,
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applicant .
Richardson, J. et al., "Controlling low temperature aqueous
synthesis of ZnO. 1. Thermodynamic analysis," Crystal Growth &
Design, 2009, vol. 9, No. 6, 2570-2575. cited by applicant .
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on GaN LEDs," SSLEC Annual Review Nov. 5, 2009. cited by applicant
.
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diodes with periodic textured Ga-doped ZnO transparent contact
layer," Applied Physics Letters 90, 263511 (2007). cited by
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ohmic contacts to p-type GaN," Applied Physics Letters, vol. 83,
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solution," Applied Physics Express 2 (2009) 042101. cited by
applicant .
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Materials , vol. 2, Dec. 2003, 821. cited by applicant.
|
Primary Examiner: Ingham; John C
Attorney, Agent or Firm: Gates & Cooper LLP
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims priority under 35 U.S.C. .sctn.119(e) to
the following co-pending and commonly-assigned U.S. Provisional
Patent Applications:
U.S. Provisional Patent Application Ser. No. 61/257,812,entitled
"HIGH BRIGHTNESS LIGHT EMITTING DIODE COVERED BY ZINC OXIDE LAYERS
ON MULTIPLE SURFACES GROWN IN LOW TEMPERATURE AQUEOUS SOLUTION,"
filed on Nov. 3, 2009,by Daniel B. Thompson, Jacob J. Richardson,
Ingrid Koslow, Jun Seok Ha, Frederick F. Lange, Steven P. DenBaars,
and Shuji Nakamura,;
U.S. Provisional Application Ser. No. 61/257,811,filed on Nov. 3,
2009,by Jacob J. Richardson, Daniel B. Thompson, Ingrid Koslow, Jun
Seok Ha, Frederick F. Lange, Steven P. DenBaars, and Shuji
Nakamura, entitled "A LIGHT EMITTING DIODE STRUCTURE UTILIZING ZINC
OXIDE NANOROD ARRAYS ON ONE OR MORE SURFACES, AND A LOW COST METHOD
OF PRODUCING SUCH ZINC OXIDE NANOROD ARRAYS,"; and
U.S. Provisional Application Ser. No. 61/257,814, filed on Nov. 3,
2009, by Daniel B. Thompson, Jacob J. Richardson, Steven P.
DenBaars, Frederick F. Lange, and Jin Hyeok Kim, entitled "LIGHT
EMITTING DIODES WITH ZINC OXIDE CURRENT SPREADING AND LIGHT
EXTRACTION LAYERS DEPOSITED FROM LOW TEMPERATURE AQUEOUS
SOLUTION,";
which applications are incorporated by reference herein.
This application is related to the following co-pending and
commonly-assigned U.S. patent applications:
U.S. Utility patent application Ser. No. 12/761,246, filed on Apr.
15, 2010, by Jacob J. Richardson and Frederick F. Lange, entitled
"LOW TEMPERATURE CONTINUOUS CIRCULATION REACTOR FOR THE AQUEOUS
SYNTHESIS OF ZnO FILMS, NANOSTRUCTURES, AND BULK SINGLE CRYSTALS,"
which application claims priority under 35 U.S.C. .sctn.119(e) to
and commonly-assigned U.S. Provisional Patent Application Ser. No.
61/169,633, filed on Apr. 15, 2009, by Jacob J. Richardson and
Frederick F. Lange, entitled "LOW TEMPERATURE CONTINUOUS
CIRCULATION REACTOR FOR THE AQUEOUS SYNTHESIS OF ZnO FILMS,
NANOSTRUCTURES, AND BULK SINGLE CRYSTALS,";
U.S. Utility application Ser. No. 12/938,872,filed on Nov. 3,2010,
by Jacob J. Richardson, Daniel B. Thompson, Ingrid Koslow, Jun Seok
Ha, Frederick F. Lange, Steven P. DenBaars, and Shuji Nakamura,
entitled "A LIGHT EMITTING DIODE STRUCTURE UTILIZING ZINC OXIDE
NANOROD ARRAYS ON ONE OR MORE SURFACES, AND A LOW COST METHOD OF
PRODUCING SUCH ZINC OXIDE NANOROD ARRAYS,", which application
claims priority under 35 U.S.C. .sctn.119(e) to and
commonly-assigned U.S. Provisional Application Ser. No. 61/257,811,
filed on Nov. 3, 2009, by Jacob J. Richardson, Daniel B. Thompson,
Ingrid Koslow, Jun Seok Ha, Frederick F. Lange, Steven P. DenBaars,
and Shuji Nakamura, entitled "A LIGHT EMITTING DIODE STRUCTURE
UTILIZING ZINC OXIDE NANOROD ARRAYS ON ONE OR MORE SURFACES, AND A
LOW COST METHOD OF PRODUCING SUCH ZINC OXIDE NANOROD ARRAYS,";
and
U.S. Utility application Ser. No. 12/939,044, filed on Nov. 3,2010,
by Daniel B. Thompson, Jacob J. Richardson, Steven P. DenBaars,
Frederick F. Lange, and Jin Hyeok Kim, entitled "LIGHT EMITTING
DIODES WITH ZINC OXIDE CURRENT SPREADING AND LIGHT EXTRACTION
LAYERS DEPOSITED FROM LOW TEMPERATURE AQUEOUS SOLUTION,", which
application claims priority under 35 U.S.C. .sctn.119(e) to and
commonly-assigned U.S. Provisional Application Ser. No. 61/257,814,
filed on Nov. 3, 2009, by Daniel B. Thompson, Jacob J. Richardson,
Steven P. DenBaars, Frederick F. Lange, and Jin Hyeok Kim, entitled
"LIGHT EMITTING DIODES WITH ZINC OXIDE CURRENT SPREADING AND LIGHT
EXTRACTION LAYERS DEPOSITED FROM LOW TEMPERATURE AQUEOUS
SOLUTION,";
which applications are incorporated by reference herein.
Claims
What is claimed is:
1. A method of fabricating a III-Nitride based Light Emitting Diode
(LED) with improved light extraction efficiency, comprising:
growing one or more Zinc Oxide (ZnO) layers on one or more
non-polar or semi-polar III-nitride light transmitting surfaces of
a III-Nitride based LED, from an aqueous solution containing
dissolved Zn(II) and by means of a chemical reaction involving the
dissolved Zn(II), wherein conditions during the growing control
crystal orientation of the ZnO layers such that the ZnO layers form
on the light transmitting surfaces in contact with the aqueous
solution and are non-epitaxial with respect to the underlying
non-polar or semi-polar III-nitride light transmitting surface.
2. The method of claim 1, wherein a maximum temperature of the
aqueous solution is less than a boiling point of the aqueous
solution.
3. The method of claim 1, wherein the dissolved Zn(II) in the
aqueous solution is supplied by dissolving a water soluble salt of
Zn(II).
4. The method of claim 1, wherein the dissolved Zn(II) in the
aqueous solution is supplied by dissolving ZnO.
5. The method of claim 1, wherein the aqueous solution is a growth
solution, and the growing of the ZnO layers is performed in one or
more steps, such that, in any one or more of the steps, the
reaction of the dissolved Zn(II) to form the ZnO layers is caused,
intensified, or otherwise controlled by an increase in a
temperature of the growth solution, or by a change in a pH of the
growth solution, or by an increase in temperature of the growth
solution and a change in pH of the growth solution.
6. The method of claim 1, wherein a morphology of the ZnO layers
produced is modified by an addition of additives to the aqueous
solution, wherein the additives include one or more of the
following: metal citrate salts, citric acid, surfactants, polymers,
biomolecules, or other molecules that interact with a surface of
ZnO or the ZnO layers.
7. The method of claim 1, wherein one or more of the ZnO layers are
grown with a two-step process, wherein a first step of the two-step
process includes deposition of a seed layer, and a second step of
the two-step process includes conversion of the seed layer into a
thicker ZnO layer by growing in the aqueous solution, wherein the
ZnO layers include the seed layer and the thicker ZnO layer.
8. The method of claim 7, wherein the deposition of the seed layer
is by depositing a solution comprising a Zn(II) precursor,
dissolved in a solvent, to create a precursor film, and then
heating the precursor film to pyrolize the Zn(II) precursor and
crystallize the ZnO seed layer.
9. The method of claim 1, further comprising controlling a
morphology of the ZnO layers by creating one or more roughened,
patterned, or structured surfaces of the ZnO layers that are
suitable for enhancing the light extraction of light emitted by the
LED.
10. The method of claim 9, wherein the roughened, patterned, or
structured surfaces of the ZnO layers are created during the step
of synthesizing the ZnO layers.
11. The method of claim 9, wherein the creation of the roughened,
patterned, or structured surfaces is created by one or more steps
involving removal of material from the preformed ZnO layers by
means of physical or chemical etching.
12. The method of claim 1, further comprising selecting conditions
wherein: one or more of the ZnO layers include one or more
epitaxial ZnO layers covering on at least one of non-polar or
semi-polar III-Nitride light transmitting surfaces, and one or more
of the ZnO layers include one or more polycrystalline ZnO layers,
on at least one of the non-polar or semi-polar III-nitride light
transmitting surfaces, with a preferential crystalline grain
texture such that, on average, ZnO crystals in the polycrystalline
ZnO layers are oriented with their [0001] c-direction perpendicular
to the covered light transmitting surfaces.
13. The method of claim 1, wherein the III-Nitride LED is a grown
on a bulk Gallium Nitride (GaN) substrate.
14. The method of claim 1, wherein the LED further comprises: a
III-nitride n-type layer; a III-nitride p-type layer; a III-nitride
active layer, for emitting light, between the n-type layer and the
p-type layer, wherein: (1) a plurality of the light transmitting
surfaces, wherein the light transmitting surfaces include a bottom
surface of the LED, sidewalls of the LED, and a top surface of the
LED, and (2) at least one of the ZnO layers is on each of the light
transmitting surfaces and the ZnO layers increase light extraction
from the LED.
15. The method of claim 1, wherein the LED is a non-polar
III-nitride LED.
16. The method of claim 1, wherein the LED is a semi-polar
III-nitride LED.
17. The method of claim 1, wherein the epitaxial ZnO layers include
a current spreading layer.
18. A method of fabricating a III-Nitride based Light Emitting
Diode (LED) with improved light extraction efficiency, comprising:
growing one or more Zinc Oxide (ZnO) layers from an aqueous
solution on one or more non-polar or semi-polar III-nitride light
transmitting surfaces of a III-Nitride based LED, wherein:
conditions during the growing control crystal orientation of the
ZnO layers such that the ZnO layers are epitaxial or non-epitaxial
with respect to underlying non-polar or semi-polar III-nitride
light transmitting surface, and one or more of the ZnO layers
include one or more polycrystalline ZnO layers, on at least one of
the non-polar or semi-polar III-nitride light transmitting
surfaces, with a preferential crystalline grain texture such that,
on average, ZnO crystals in the polycrystalline ZnO layers are
oriented with their [0001] c-direction perpendicular to the
underlying non-polar or semi-polar III-nitride surface.
19. The method of claim 18, wherein one or more of the
polycrystalline ZnO layers are roughened in a way that increases
light extraction from the LED.
20. The method of claim 18, wherein the ZnO layers are epitaxially
grown on one or more of the light transmitting non-polar or
semi-polar III-Nitride surfaces of the LED.
21. A method of fabricating a III-Nitride based Light Emitting
Diode (LED) with improved light extraction efficiency, comprising:
growing one or more Zinc Oxide (ZnO) layers from an aqueous
solution on one or more non-polar or semi-polar III-nitride
lighttransmitting surfaces of a III-Nitride based LED, wherein:
conditions during the growing control crystal orientation of the
ZnO layers such that the ZnO layers are epitaxial or non-epitaxial
with respect to the underlying non-polar or semi-polar III-nitride
light transmitting surface, and the ZnO layers include an epitaxial
ZnO film and a non-epitaxial ZnO film.
22. The method of claim 21, further comprising growing the ZnO
layers from the aqueous solution containing dissolved Zn(II), by
means of a chemical reaction involving dissolved Zn(II), wherein:
the ZnO layers form on the light transmitting surfaces in contact
with the aqueous solution, the reaction of the dissolved Zn(II) to
form the ZnO layers is caused, intensified, or otherwise controlled
by an increase in a temperature of the growth solution or by a
change in a pH of the growth solution, or by an increase in
temperature of the growth solution and a change in pH of the growth
solution, one or more of the ZnO layers are grown with a two-step
process, wherein a first step of the two-step process includes
deposition of a seed layer and a second step of the two-step
process includes conversion of the seed layer into a thicker ZnO
layer by growing in the aqueous solution, wherein the ZnO layers
include the seed layer and the thicker ZnO layer, and the
deposition of the seed layer is by depositing a solution comprising
a Zn(II) precursor, dissolved in a solvent, to create a precursor
film, and then heating the precursor film to pyrolize the Zn(II)
precursor and crystallize the ZnO seed layer.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to enhancing light extraction from high
brightness light emitting diodes (LEDs).
2. Description of the Related Art
(Note: This application references a number of different
publications as indicated throughout the specification by one or
more reference numbers within brackets, e.g., [x]. A list of these
different publications ordered according to these reference numbers
can be found below in the section entitled "References." Each of
these publications is incorporated by reference herein.)
It has been reported that transparent conductive oxide (TCO) films
consisting of indium-tin-oxide (ITO) [1,2], Zinc Oxide (ZnO) [3],
or aluminum-doped-zinc-oxide (AZO) [4] improve external quantum
efficiency when applied to the surface of GaN LEDs. In addition, it
has been shown that low temperature aqueous deposition can be used
to fabricate ZnO current spreading layers on GaN LEDs, see [11].
Compared to a conventional thin metal current spreading layer, the
ZnO layer deposited from low temperature aqueous solution improved
light power output by over 90%. It has also been shown that several
methods of transparent conductive oxide (TCO) layer surface
roughening can also improve light extraction from LEDs [5,6].
However, all of this prior research has been conducted using TCO
layers deposited only on Ga faced c-plane, p-type GaN surfaces of
LEDs.
SUMMARY OF THE INVENTION
To improve the light extraction efficiency of LEDs, transparent
conductive oxides (TCOs) with high refractive indices, such as
indium-tin-oxide (ITO), ZnO, aluminum-doped-zinc-oxide (AZO), are
widely used. When applied to the surface of an LED, films of these
materials increase the probability of light escaping the LED
through the TCO. The probability that a photon will pass from a
high index of refraction material to a material with a lower, yet
close, index of refraction is significantly improved over the
probability that a photon will pass from a high index material to a
low index material. The refractive index of Zn, which is
approximately 2.1, is between that of III-Nitride materials used
for LEDs, e.g. n.sub.GaN=2.5, and all the currently used
encapsulants known to the inventors. Thus, a layer of ZnO on a
surface of a III-Nitride LED is expected to aid photons in escaping
that LED surface.
Using the Fresnel equations, it is possible to calculate the
enhancement in light transmission from a LED surface using a layer
of intermediate refractive index, e.g., a ZnO layer. As an example,
a photon with a 450 nm wavelength and normal incidence on a GaN and
ZnO interface has a 95% probability of passing through the ZnO and
then into an encapsulant of index 1.4, while a photon with normal
incidence on an interface between GaN and the same encapsulant, has
a only a 92% probability of passing through the GaN into the
encapsulant. Going from normal incidence to shallower angles of
incidence, the enhancement in the probability of transmission
becomes even greater. Without a ZnO layer, the probability
transmission from the GaN directly into the encapsulant drops to 0%
at incidence angles higher than the critical angle of 34.degree..
When a ZnO layer is present, the critical angle is increased to
42.degree.. In total, these effects are expected to result in a 27%
increase in transmission through a GaN/ZnO/encapsulant surface as
compared to a GaN/encapsulant surface.
Thus, TCOs of intermediary refractive indices, i.e. an index
between that of the most external III-Nitride layer and the
material immediately surrounding the TCO, allow more of the photons
produced in the LED's active region to pass through the external
surface without being reflected back into the LED. In state of the
art LEDs, TCOs of this nature are usually deposited on the p-type
GaN terminated (0001) surface of the LED. However, the active
region of the LED emits light in all directions. This means a large
portion of the light generated is not immediately directed through
the p-type GaN surface. Light not directed to the TCO coated
surface will have a smaller chance of escape and will be more
likely to be internally reflected several times before escaping the
LED. The longer path length of these internally reflected photons
makes for a greater probability of those photons being reabsorbed.
This in turn lowers the LED's external quantum efficiency.
Depositing high refractive index TCO layers on the other surfaces
of the LED would allow a larger number of photons to escape without
being internally reflected, thus increasing the external quantum
efficiency of the LED. However, most of the techniques typically
used for depositing TCO films are either not capable of depositing
on multiple surfaces of the LED, or doing so would be cost
prohibitive. This has limited the use of TCO layers for enhancing
light extraction from multiple LED surfaces.
In this disclosure, the present invention describes LED structures
which utilize TCO layers on one or more surfaces. The TCO used is
ZnO deposited by a low temperature aqueous route. This is a low
cost, flexible method that can grow ZnO layers before or after the
LED chip fabrication processing. The present invention also
describes how these ZnO layers can be etched to produce a surface
texture to further increase the light extraction from the LEDs. The
high electrical conductivity, good thermal conductivity, high light
transmission, and surface texturing possible with ZnO layers,
combined with the low cost and simplicity of low temperature
aqueous processing will be useful for developing low cost, high
light output GaN LED devices. This method can be used to deposit
ZnO layers on both vertical and lateral type LEDs, using either
bulk GaN or heteroepitaxial substrates.
To overcome the limitations in the prior art, and to overcome other
limitations that will become apparent upon reading and
understanding the present specification discloses an optoelectronic
device, comprising a high brightness III-Nitride based LED, wherein
multiple surfaces (e.g., light transmitting surfaces) of the LED
are covered by one or more Zinc Oxide (ZnO) layers.
Multiple surfaces of the LED may be covered by the ZnO layers. The
ZnO layers may encapsulate or surround the LED.
One or more of the light transmitting surfaces, covered by the ZnO
layers, may be different from a p-type III-Nitride c-plane surface
(e.g., different from a Ga faced c-plane surface of p-type
III-nitride). One or more of the light transmitting surfaces may
include at least one III-Nitride semipolar or nonpolar surface. One
or more of the light transmitting surfaces may include at least one
non-III-Nitride surface.
One or more of the ZnO layers may include at least one epitaxial
ZnO layer covering or grown on a III-Nitride surface of the
LED.
A surface different from a Ga faced c-plane surface may be an N
faced c-plane surface. A surface different from a Ga faced c-plane
surface may be a III-Nitride non-polar or semi-polar plane surface.
The surface different from a c-plane surface may be a surface that
is not a III-Nitride surface.
A p-type III-Nitride surface of the device may also be a light
transmitting surface and be covered by at least one of the ZnO
layers. Or, a p-type III-Nitride surface of the device may not be a
light transmitting surface and is covered by a reflective p-contact
layer.
The III-Nitride LED may be a conventional LED grown on a
heteroepitaxial substrate or a homoepitaxial LED grown on a bulk
GaN substrate.
The III-Nitride LED may be a lateral (mesa) or vertical
architecture type device.
The ZnO layers on one or more of the surfaces may be one or more
epitaxial ZnO layer, wherein the epitaxy is with a III-Nitride
layer or layers.
One or more of the ZnO layers may include one or more
polycrystalline ZnO layers with a preferential crystalline grain
texture such that, on average, ZnO crystals in the polycrystalline
ZnO layers are oriented with their [0001] c-direction perpendicular
to the covered light transmitting surfaces.
The LED may further comprise an n-type layer; a p-type layer; an
active layer, for emitting light, between the n-type layer and the
p-type layer, wherein (1) the multiple surfaces are light
transmitting surfaces, for transmitting the emitted light, and
include a bottom surface of the LED, sidewalls of the LED, or a top
surface of the LED, and (2) at least one of the ZnO layers is on a
light transmitting surfaces, and the ZnO layers increase light
extraction from the LED.
The present invention further discloses a method of fabricating an
optoelectronic device with improved light extraction efficiency,
comprising covering multiple surfaces of a III-Nitride LED with one
or more Zinc Oxide (ZnO) layers. The covering may include growing
the ZnO.
The growth of the ZnO layers may be performed in one or more steps,
wherein at least one step involves growing ZnO from an aqueous
solution. The aqueous solution used for ZnO growth may contain
dissolved Zn(II) provided by dissolving a soluble Zn(II) salt, or
by dissolving ZnO. The growth from aqueous solution may be
performed at a temperature of less than 100.degree. C.
Alternatively, the growth from aqueous solution may be performed at
a temperature less than the boiling point of the aqueous solution,
wherein that boiling point may be less than or greater than
100.degree. C. The ZnO layers may be formed from aqueous solution
by a chemical reaction of the dissolved Zn(II) to form ZnO, wherein
the chemical reaction is caused, intensified, or otherwise
controlled by an increase in the temperature of the growth
solution, or by a change in the pH of the growth solution. The
conditions, e.g., temperature, constituent concentrations, or pH,
of the aqueous growth step may be used to control the crystal
orientation or texture, thickness, or surface morphology of the ZnO
layer produced.
The growth of the ZnO layers may include a seed layer deposition
step, where a thin crystalline ZnO seed layer is formed on one or
more surfaces of the III-Nitride LED. A seed layer may be deposited
from aqueous solution as described above. Alternatively, a seed
layer may be deposited by first depositing a Zn(II) precursor,
dissolved in a solvent, on one or more surfaces to form precursor
films, followed by annealing the precursor film to convert the
precursor film to a crystalline ZnO film. Alternatively, the seed
may be deposited using a vapor phase method, e.g., sputtering,
evaporation, or chemical vapor deposition.
The growth of ZnO layers may comprise converting one or more seed
layers to a thicker ZnO layer, by growing further ZnO on the seed
layer from an aqueous solution, as described above. The conditions
used for the deposition of the Zn(II) precursor film, or for the
conversion of the Zn(II) precursor film into a crystalline ZnO
layer, may be used to control the crystal orientation or texture,
thickness, or surface morphology of final ZnO layers.
One or more of the ZnO layers may be roughened in a way that
increases light extraction from the LED. The crystal orientation or
texture, thickness, or surface morphology of the ZnO layers grown
on light transmitting surfaces of the III-Nitride LED may be
controlled to enhance the light extraction from those surfaces of
the LED.
BRIEF DESCRIPTION OF THE DRAWINGS
Referring now to the drawings in which like reference numbers
represent corresponding parts throughout:
FIGS. 1(a)-(e) show cross-sectional schematics of the conventional
LED structures with a TCO layer on p-type GaN layer.
FIG. 2 shows the calculated probability of light transmission as a
function of incidence angle through a GaN/Polydimethylsiloxane
(PDMS) encapsulant interface and through a GaN/ZnO/PDMS encapsulant
double interface.
FIGS. 3(a)-(f) show cross-sectional schematics of the new LED
structures with ZnO layer on multiple surface planes, according to
the present invention.
FIG. 4(a)-(d) shows cross-sectional schematics of the new LED
structures with ZnO layers, according to another embodiment of the
present invention.
FIG. 5 is a flowchart illustrating a method of fabricating ZnO
layers on multiple surfaces of a III-Nitride LED, according to one
embodiment of the present invention.
FIG. 6 is a flowchart illustrating a method of fabricating ZnO
layers on multiple surfaces of a III-Nitride LED, according to
another embodiment of the present invention.
FIG. 7 shows a scanning electron microscope image of a
polycrystalline ZnO film, with a preferred c-direction surface
normal orientation that has been grown on a non-polar surface of a
bulk GaN substrate.
DETAILED DESCRIPTION OF THE INVENTION
In the following description of the preferred embodiment, reference
is made to the accompanying drawings which form a part hereof, and
in which is shown by way of illustration a specific embodiment in
which the invention may be practiced. It is to be understood that
other embodiments may be utilized and structural changes may be
made without departing from the scope of the present invention.
Overview
The purpose of this invention is to improve the performance of
III-Nitride (e.g., GaN) LEDs through the addition of ZnO layers to
the LED's surfaces. These layers can improve the light extraction,
heat dissipation, and current distribution of the device. Due to
energy concerns, efficient LED lighting technology is of current
and expanding importance. However, the cost of LED lighting remains
high and is a major impediment to the greater implementation of the
technology. This invention allows the power output of LEDs to be
increased, for example, by over 90%, without substantially
increasing the cost of the device.
Technical Description
Nomenclature
III-nitrides may be referred to as group III-nitrides, nitrides, or
by (Al,Ga,In)N, AlInGaN, or Al.sub.(1-x-y)In.sub.yGa.sub.xN where
0<x<1 and 0<y<1, for example.
These terms are intended to be broadly construed to include
respective nitrides of the single species, Al, Ga, and In, as well
as binary, ternary and quaternary compositions of such Group III
metal species. Accordingly, the terms comprehend the compounds AN,
GaN, and InN, as well as the ternary compounds AlGaN, GaInN, and
AlInN, and the quaternary compound AlGaInN, as species included in
such nomenclature. When two or more of the (Ga, Al, In) component
species are present, all possible compositions, including
stoichiometric proportions as well as "off-stoichiometric"
proportions (with respect to the relative mole fractions present of
each of the (Ga, Al, In) component species that are present in the
composition), can be employed within the broad scope of the
invention. Accordingly, it will be appreciated that the discussion
of the invention hereinafter in primary reference to GaN materials
is applicable to the formation of various other (Al, Ga, In)N
material species.
Further, (Al,Ga,In)N materials within the scope of the invention
may further include minor quantities of dopants and/or other
impurity or inclusional materials. Boron may also be included in
the III-nitride alloy.
Similarly, the terms zinc oxide or ZnO are intended to be broadly
construed to include any material where the component species Zn
and O make up the majority of the compound, and the material
retains the hexagonal Wurtzite crystal structure of ZnO. This is
inclusive of aluminum doped zinc oxide (AZO), gallium doped zinc
oxide (GZO), and indium doped zinc oxide (IZO). This also includes
materials with minor quantities of other dopants and/or other
impurity or inclusional materials, as well as materials that are
off-stoichiometric due to the presence of vacancy and interstitial
type material defects.
Current nitride technology for electronic and optoelectronic
devices employs nitride films grown along the polar c-direction.
However, conventional c-plane quantum well structures in
III-nitride based optoelectronic and electronic devices suffer from
the undesirable quantum-confined Stark effect (QCSE), due to the
existence of strong piezoelectric and spontaneous polarizations.
The strong built-in electric fields along the c-direction cause
spatial separation of electrons and holes that in turn give rise to
restricted carrier recombination efficiency, reduced oscillator
strength, and red-shifted emission.
One approach to eliminating the spontaneous and piezoelectric
polarization effects in GaN or III-nitride optoelectronic devices
is to grow the devices on nonpolar planes of the crystal. Such
planes contain equal numbers of Ga and N atoms and are
charge-neutral. Furthermore, subsequent nonpolar layers are
equivalent to one another so the bulk crystal will not be polarized
along the growth direction. Two such families of
symmetry-equivalent nonpolar planes in GaN or III-nitride are the
{11-20} family, known collectively as a-planes, and the {1-100}
family, known collectively as m-planes.
Another approach to reducing or possibly eliminating the
polarization effects in GaN optoelectronic devices is to grow the
devices on semi-polar planes of the crystal. The term "semi-polar
planes" can be used to refer to a wide variety of planes that
possess both two nonzero h, i, or k Miller indices and a nonzero 1
Miller index. Thus, semipolar planes are defined as crystal planes
with nonzero h or k or i index and a nonzero l index in the (hkil)
Miller-Bravais indexing convention. Some commonly observed examples
of semi-polar planes in c-plane GaN heteroepitaxy include the
(11-22), (10-11), and (10-13) planes, which are found in the facets
of pits. Other examples of semi-polar planes in the wurtzite
crystal structure include, but are not limited to, (10-12),
(20-21), and (10-14). The nitride crystal's polarization vector
lies neither within such planes or normal to such planes, but
rather lies at some angle inclined relative to the plane's surface
normal. For example, the (10-11) and (10-13) planes are at
62.98.degree. and 32.06.degree. to the c-plane, respectively.
The Gallium or Ga face of GaN is the c.sup.+ or (0001) plane, and
the Nitrogen or N-face of GaN or a III-nitride layer is the c.sup.-
or (000-1) plane.
LED Structures
This invention describes an LED structure with ZnO layers deposited
on one or more surfaces other than a (0001) p-type III-Nitride
surface. The present invention further describes ZnO layers
deposited on multiple surfaces of the LED. The multiple surfaces
may include (0001) p-type GaN surfaces and surfaces other than
(0001) p-type GaN surfaces.
The present invention includes ZnO layers that have been etched, or
otherwise treated, subsequent to growth to alter the surface
roughness or morphology of the film. The purpose of the ZnO layers
in such a structure is to enhance light extraction, and/or help
dissipate heat, and/or serve as a transparent current spreading
layer. It is well established that the basal planes of ZnO tend to
etch into hexagonal pits and pyramids, and that such etched
structures serve to enhance light extraction when applied to the
surfaces of LED devices. It is therefore useful to be able to grow
c-direction oriented ZnO films on different LED surfaces. In other
situations, it may be beneficial for the ZnO layers to be epitaxial
with respect to the underlying crystal. Epitaxial films should
display higher electronic mobility, thermal conductivity, and
optical transparency.
Here, the present invention describes low cost solution based
processes for synthesizing c-direction oriented ZnO layers and/or
epitaxial ZnO layers on multiple surfaces of a LED structure.
FIGS. 1(a)-(c) show the schematic diagrams of conventional
structured LEDs 100, comprising an n-type GaN layer 102 on a
sapphire 104 or GaN substrate (e.g., GaN bulk) 106, an active layer
108 or region on the n-type GaN layer 102, and a p-type GaN layer
(p-GaN layer) 110 on the active layer 108.
FIG. 1(a) is an LED 100 using a TCO layer 112 on top of the p-type
GaN layer 110, with a heteroepitaxial substrate 104. FIGS. 1(b) and
(c) are LEDs grown on bulk GaN substrates 106, and using a TCO
layer 112 on a p-type GaN layer 110. The backside surface of the
GaN substrate 106 may be roughened 116 to enhance light
extraction.
Also shown are the n-contact 118 on the n-type GaN layer 102, the
p-contact 120 on the TCO layer 112, and an n-contact 122 to the GaN
bulk 106. FIG. 1(d)-(e) illustrate roughened TCO layers 112.
FIG. 2 shows the calculated probability of light transmission as a
function of incidence angle through a GaN/Polydimethylsiloxane
(PDMS) encapsulant interface (critical angle
.theta..sub.c=34.degree.) and through a GaN/ZnO/PDMS encapsulant
double interface (critical angle .theta..sub.c=42.degree.), showing
a 27% increase in extraction using the ZnO layer.
FIGS. 3 and 4 show schematic diagrams of LED devices utilizing ZnO
films on multiple LED surfaces. Different variations on these types
of structures can also be made, and multiple processing methods
could be used to realize these types of structures. Here, the
present invention shows results from LED devices on bulk GaN
substrates. These LEDs were fully fabricated and the ZnO layers
were deposited as a final step.
FIG. 3 and FIG. 4 show the schematic diagrams of the new present
invention. A difference from the conventional LED 100 structures is
that in the present invention, multiple surfaces 302a, 302b, 302c,
and 302d of the LED 300 are covered by ZnO layers 304a, 304b, 304c,
and 304d, where at least one of those surfaces is different from
the p-type surface 302c. The ZnO layers 304a-d may encapsulate or
surround the LED.
In the embodiments of FIG. 3(a)-(f), the LED 300 comprises a
III-nitride n-type layer 306; a III-nitride p-type layer 308; a
III-nitride active layer 310, for emitting light, between the
n-type layer 306 and the p-type layer 308, wherein (1) a plurality
of multiple surfaces 302a, 302b, 302c, 302d are light transmitting
surfaces, for transmitting the emitted light, and include a bottom
surface 302a of the LED 300, sidewalls 302b of the LED, a top
surface 302c of the LED, and a surface of the n-type layer 302d,
and (2) at least one of the ZnO layers 304a-d is on each of the
light transmitting surfaces and the ZnO layers 304a-d increase
light extraction from the LED. Also shown is the LED grown on a
heteroepitaxial substrate 312 or a bulk GaN 314 substrate.
The top surface 302c is the surface of the p-type layer 308.
The sidewalls 302b may include the sidewalls the substrate 314, the
n-type layer 306, the active layer 310 and/or the p-type layer
308.
A p-type contact 316 (p-contact) ohmically contacts the ZnO layer
304c, an n-type contact 318a (n-contact) ohmically contacts the
n-type layer 306 and/or the ZnO layer 304d, or an n-contact 318b
ohmically contacts the bulk GaN 314 and/or the ZnO layer 304a, and
a passivation layer 320 is on the sidewalls of the n-type layer
306, the p-type layer 308, and the active region 310.
In the case of FIG. 3(d), (e) and (f), the ZnO layers 304a-d on all
the surface planes 302a-d have a roughness or structuring 322a by
etching to increase light extraction efficiency. However, the ZnO
layers 304a-d may also be a planar surface 322b. At least one of
the surfaces of the substrate, e.g. the bottom light extracting
surface of the GaN bulk 314, may also be roughened 324 to enhance
light extraction.
The enhancement of extraction may be by scattering, reduced total
internal reflection, diffraction, or photonic crystal effects, for
example. The roughening or structuring 322a of the ZnO layer may
create structures in the ZnO layer surface having dimensions
sufficiently close to a wavelength of the light emitted by the LED,
for example, so that the light may be scattered, diffracted,
reflected, or otherwise interact electromagnetically with the
structures.
A ZnO layer 304a-d may be a polycrystalline layer with a grain
texture providing an average crystal orientation of the grains with
the [0001] c-direction perpendicular to the light transmitting
surfaces 302a such that a ZnO layer 304a-d has a surface comprising
of the (0001) c-plane of ZnO.
FIG. 4(a)-(d) are cross-sectional schematics of an optoelectronic
device, comprising a III-Nitride LED 400, wherein the LED 400
device includes an n-type GaN layer 402 grown on a GaN bulk
substrate 404, a III-Nitride active layer 406 grown on the n-type
GaN layer 402, a p-type GaN layer 408 grown on the active layer
406, a reflective p-type contact 410 deposited on the p-type GaN
408, a metal support 412 on the reflective p-contact 410, one or
more ZnO layers 414 on the n-type GaN 402 and/or on the GaN bulk
404, and an n-type pad 416 on the ZnO layers 414 (FIG. 4(a) and
FIG. 4(c)), or on the GaN bulk 404 (FIG. 4(b) and FIG. 4(d)). The
ZnO layers 414 have a planar surface 418 (FIG. 4(a)-(b) or a
roughened surface 420 (FIG. 4(c)-(d)).
Thus, FIGS. 3(a)-(f) and FIGS. 4(a)-(d) illustrate an
optoelectronic device, comprising a III-Nitride LED 300, 400
wherein multiple light transmitting surfaces 302a-c of the LED 300
are covered by one or more ZnO layers 304a, 414. One or more of the
light transmitting surfaces 302a, covered by the ZnO layers 304a,
include surfaces different from a p-type III-Nitride c-plane
surface. One or more of the light transmitting surfaces, covered by
the ZnO layers 414, may include at least one III-Nitride semipolar
or nonpolar surface, at least one non-III-Nitride surface 302a,
and/or at least one epitaxial ZnO layer 304c covering a III-Nitride
surface 302c of the LED 300.
The III-Nitride LED may be a conventional LED grown on a
heteroepitaxial substrate 312 or a homoepitaxial LED grown on a
bulk GaN substrate 314. The III-Nitride LED may be a lateral (mesa)
architecture type device 300 (as shown in FIGS. 3(a)-(b), or a
vertical architecture type device 400 (as shown in FIGS.
4(a)-(d)).
A p-type III-Nitride surface 302c of the device may be a light
transmitting surface and may be covered by at least one of the ZnO
layers 304c. A p-type III-Nitride surface of a p-type layer 408 of
the device may not be a light transmitting surface and may be
covered by a reflective p-contact layer 410.
One or more of the ZnO layers 414 may be roughened 420 in a way
that increases light extraction from the LED.
The light transmitting surfaces are typically transparent to
transmit light having the wavelength emitted by LED.
ZnO Synthesis
ZnO shares the wurtzite crystal structure and is well lattice
matched with GaN, a fact that will encourage the epitaxial growth
of ZnO on any crystallographic plane of GaN. This is beneficial
when attempting to grow c-direction oriented ZnO films on the basal
planes of GaN, but makes the growth of c-direction oriented ZnO on
the other planes of GaN difficult. For these planes, the growth of
a c-direction oriented film requires inhibiting epitaxy. When
epitaxial growth is inhibited, ZnO films with a preferential
c-direction orientation will tend to result. This occurs for
several reasons. First, non-epitaxial ZnO will tend to nucleate
with a basal plane orientation to minimize surface energy. These
oriented nuclei then grow into oriented grains. Secondly,
c-direction oriented grains will tend to dominate in a thicker film
due to the fact that non-oriented grains are self terminating.
Because ZnO tends to grow faster in the [0001] direction,
non-oriented grains will quickly run into adjacent grains, ending
their growth. Meanwhile oriented grains can continue to grow normal
to the surface, unimpeded. Conversely, when synthesizing epitaxial
ZnO films, the nucleation of epitaxial seeds must be encouraged
rather than inhibited. Once an epitaxial seed layer has been
nucleated, further nucleation should then be minimized allowing the
epitaxial seeds to grow into a thicker coalesced ZnO film.
FIG. 5 illustrates a method of fabricating a III-Nitride based LED
with improved light extraction efficiency.
Block 500 represents covering multiple surfaces (e.g., III-Nitride
or non-III-Nitride light transmitting surfaces) of the III-Nitride
LED with one or more ZnO layers. The covering may include growing
one or more ZnO layers on one or more light transmitting surfaces
of a III-Nitride based LED, wherein the layers are grown on at
least one light transmitting surface that is different from a
p-type III-Nitride c-plane surface of the LED. The covering may
include either epitaxial or polycrystalline (non-epitaxial) ZnO
films on any of the multiple surfaces. The ZnO may be grown using
one or more growth steps ZnO is grown from an aqueous solution
containing dissolved Zn(II)[13]. The growth solution may be at a
temperature less than the boiling point of the aqueous solution,
such that the aqueous solution is a liquid aqueous solution. One or
more of the ZnO layers may include one or more polycrystalline ZnO
layers with a preferential crystalline grain texture such that, on
average, ZnO crystals in the polycrystalline ZnO layers are
oriented with their [0001] c-direction perpendicular to the covered
light transmitting surfaces.
The growing may comprise growing the ZnO layers from an aqueous
solution containing dissolved Zn(II), by means of a chemical
reaction involving dissolved Zn(II), wherein the ZnO layers form on
the light transmitting surfaces in contact with the aqueous
solution.
A maximum temperature of the aqueous solution may be less than a
boiling point of the aqueous solution. The dissolved Zn(II) in the
aqueous solution may be supplied by dissolving a water soluble salt
of Zn(II) and/or by dissolving ZnO.
The aqueous solution may be a growth solution, and the growing of
the ZnO layers may be performed in one or more steps, such that, in
any one or more of the steps, the reaction of the dissolved Zn(II)
to form the ZnO layers is caused, intensified, or otherwise
controlled by an increase in a temperature of the growth solution,
or by a change in a pH of the growth solution.
A morphology of the ZnO layers produced may be modified by an
addition of additives to the aqueous solution, wherein the
additives include one or more of the following: metal citrate
salts, citric acid, surfactants, polymers, biomolecules, or other
molecules that interact with a surface of ZnO or the ZnO
layers.
Block 502 represents controlling a morphology of the ZnO layers by
creating one or more roughened, patterned, or structured surfaces
of the ZnO layers that are suitable for enhancing the light
extraction of light emitted by the LED. The crystal orientation or
texture, thickness, or surface morphology of the ZnO layers grown
on light transmitting surfaces of the III-Nitride LED may be
controlled to enhance the light extraction from those surfaces of
the LED. The creation of the roughened, patterned, or structured
surfaces may include one or more steps involving removal of
material from the preformed ZnO layers by means of physical or
chemical etching. The roughened, patterned, or structured surfaces
of the ZnO layers may be created during the process of synthesizing
the ZnO layers in Block 500.
The conditions, e.g., temperature, constituent concentrations, or
pH, of the aqueous growth step may be used to control the crystal
orientation or texture, thickness, or surface morphology of the ZnO
layer produced.
Block 504 represent the end result of the method, an optoelectronic
device such as an LED, wherein one or more epitaxial ZnO layers are
grown on one or more of the light transmitting III-Nitride multiple
surfaces of the LED. The multiple surfaces may comprise at least
one surface different from a Ga faced c-plane surface of a p-type
layer of the III-Nitride based LED. A surface different from a Ga
faced c-plane surface may be an N faced c-plane surface. A surface
different from a Ga faced c-plane surface may be a III-Nitride
non-polar or semi-polar plane surface. The surface different from a
c-plane surface may be a surface that is not a III-Nitride
surface.
The ZnO layers on one or more of the surfaces may be one or more
epitaxial ZnO layers grown on, or covering, one or more III-Nitride
or light transmitting III-Nitride surfaces of the LED, wherein the
epitaxy is with a III-Nitride layer or layers.
The ZnO layers on one or more of the surfaces may be
polycrystalline and textured such that on average the c-direction
[0001] of the ZnO crystals of that layer are perpendicular to that
surface.
The LED may further comprise an n-type layer; a p-type layer; an
active layer, for emitting light, between the n-type layer and the
p-type layer, wherein (1) the multiple surfaces are light
transmitting surfaces, for transmitting the emitted light, and
include a bottom surface of the LED, sidewalls of the LED, or a top
surface of the LED, and (2) at least one of the ZnO layers is on a
light transmitting surfaces, and the ZnO layers increase light
extraction from the LED.
In other embodiments, the growing of Block 500 occurs in two steps,
as represented by FIG. 6.
Block 600 represents growing or depositing ZnO seed layers (e.g.,
thin seed layers) on one or more of the multiple surfaces. The ZnO
seed layers be either epitaxial or polycrystalline (non-epitaxial)
seed layers. The deposition of the seed layer may include
depositing a solution comprising a Zn(II) precursor, dissolved in a
solvent, to create a precursor film, and then heating the precursor
film to pyrolize the Zn(II) precursor and crystallize the ZnO seed
layer.
Block 602 represents growing the ZnO layers on multiple surfaces of
a III-Nitride LED, wherein one or more layers may be grown by
growing a ZnO seed layers into thicker ZnO layer. The step may
include conversion of the thin seed layer into a thicker ZnO layer
by growing in the aqueous solution, wherein the ZnO layers include
the seed layer and the thicker ZnO layer. The conditions used for
the deposition of the Zn(II) precursor film, or for the conversion
of the Zn(II) precursor film into a crystalline ZnO layer, may be
used to control the crystal orientation or texture, thickness, or
surface morphology of final ZnO layers.
Block 604 represents the end result of the method, a device such as
an optoelectronic device comprising a high brightness III-Nitride
based LED, wherein multiple surfaces of the LED are covered by one
or more ZnO layers. The ZnO layers may comprise a ZnO film, e.g. an
epitaxial ZnO film or a polycrystalline film (non-epitaxial), on
the III-Nitride LED.
Non-Epitaxial Seed Layer Deposition
ZnO films with preferential c-direction orientation can be formed
on any arbitrary surface of an LED using a two step process. First,
a thin polycrystalline ZnO seed layer may be deposited on the
surfaces (Block 600). In one embodiment, a solution based, ZnO
precursor decomposition method is used to accomplish this. A Zn(II)
salt or other Zn(II) precursor is dissolved in suitable solvent
along with any other additives for modifying the characteristics of
the solution. For the experimental results shown here, Zn(II)
Acetate dehydrate was dissolved in ethanol at a concentration of
0.5 mol/L along with 0.5 mol/L diethanolamine. The addition of
diethanolamine serves to increase the solubility of Zn in solution,
as well modify the viscosity and drying behavior of the solution.
The precursor solution is then deposited on the desired surfaces of
the LED by a chemical solution deposition method such as spin
coating, dip coating, or spray coating. The resulting film is then
annealed, either first at an intermediate temperature to pyrolyze
the film, followed by a higher temperature crystallize the film, or
immediately at a temperature high enough to both pyrolyze the
precursor and crystalline ZnO. The deposition of precursor solution
can be repeated, before or after the pyrolysis or crystallization
steps to give greater seed layer thickness. Variations in the
conditions used will alter the properties of the resulting film.
For the results presented in FIG. 6, the precursor solution was
spin-coated onto the LED followed by a rapid thermal annealing
treatment to 600.degree. C. in an N.sub.2/O.sub.2 atmosphere to
both pyrolyze and crystallize the precursor film into a ZnO seed
layer.
Epitaxial Seed Layer Deposition
For LEDs which have surfaces displaying one or more
crystallographic orientations of a Wurtzite III-Nitride, it is
possible to deposit epitaxial ZnO layers. Growth of epitaxial ZnO
on III-Nitride can be accomplished using aqueous solution routes,
as described by Thompson et al [11] and Kim et al [7]. Although
these reports dealt with only the (0001) orientation of GaN, the
same procedure may be used for other GaN surfaces. These reports
utilize a two step approach to grow thick coalesced ZnO films. The
first step serves to form a high density of epitaxial ZnO nuclei on
the GaN surface (Block 400). In practice, this is accomplished by
preheating an aqueous solution of zinc nitrate and ammonium nitrate
to 90.degree. C., and then adding aqueous ammonia to raise the pH.
Richardson and Lange [8] describe how this procedure rapidly
increases the supersaturation of ZnO in solution, supplying the
high driving force needed to create a high density of ZnO nuclei.
In this case, the required supersaturation was created by rapidly
increasing the pH of the solution, however, a large supersaturation
can also be created by changing other conditions. Of particular
importance, Richardson and Lange [8] also show that under certain
solution conditions it is possible to create a supersaturation by
heating the solution. The specific conditions used to nucleate ZnO
on GaN will affect the properties of the ZnO layer produced. For
best results the conditions should lead to a high density of nuclei
while minimizing non epitaxial nucleation. Epitaxial nucleation is
energetically favorable, but excessively high supersaturations can
lead to non-epitaxial nucleation. The supersaturation event leading
to nucleation should also be transitory, as a continuous high
supersaturation can lead to unfavorable secondary nucleation.
Aqueous Conversion of ZnO Seed Layers to Thicker ZnO Films
After the seed layer deposition of Block 600, a second step (Block
602) is typically used to grow the ZnO seed layer into a thicker
film. This is accomplished using growth from an aqueous solution.
Growth of ZnO from aqueous solution is well known, and many
different specific solution conditions can be used for this
purpose. The specific solution conditions used will, along with the
properties of the seed layer, determine the final properties of the
ZnO film. The growth solution may contain a Zn(II) source such as a
salt or another Zn containing material that can be dissolved to
appreciable concentrations. Typically, the solution will also
contain other chemicals which serve to complex Zn, modify the pH of
the solution, and/or interact with the surface of the growing ZnO
crystals. For ZnO to deposit from a stable solution, growth may be
initiated by some change in the solution conditions. For example,
this can be achieved with a change in the pressure, pH,
temperature, or the concentrations of one or more of the solution
components. The results shown in this disclosure were obtained
using the second step of the procedure reported by Thompson et al
[11]. In this procedure, ZnO forms after a stable room temperature
solution containing Zn nitrate and ammonia is heated to 90.degree.
C. In some cases, sodium citrate is also added to the growth
solution. The citrate anions, provided by the addition of sodium
citrate, slow the growth in the c-direction and encourage a ZnO
morphology with more exposed c-plane surfaces see [9-11]. In the
case of non-epitaxial seed layers, otherwise similar growth
conditions would result in ZnO nanorod arrays without the presence
of citrate in the growth solution. Other additives may be able to
modify the growth in a similar way. For the growth of epitaxial
ZnO, the specific orientations being grown will determine if
citrate ions, or other additives, are beneficial or not.
During the conversion of the ZnO seed layers into thicker films,
all growth should occur on the preexisting seed layer. To achieve
this, the supersaturation in solution must be maintained at levels
that are high enough to lead to appreciable ZnO growth, but low
enough to minimize new nucleation. In some cases it may be possible
to reach this intermediate level of supersaturation after an
initial period of higher supersaturation. In principle, a procedure
that accomplished this would allow the nucleation of the seed layer
and the subsequent growth into a thicker film to be achieved in a
single growth step.
FIG. 7 shows a scanning electron microscope image of a
polycrystalline ZnO film, with a preferred c-direction surface
normal orientation that has been grown on a non-polar surface of a
bulk GaN substrate.
REFERENCES
The following references are incorporated by reference herein.
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[11] Daniel B. Thompson, Jacob J. Richardson, Steven P. DenBaars,
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[12] Presentation Slides given by Jacob Richardson, entitled "Low
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Annual Review for the Solid State Lighting and Energy Center
(SSLEC), University of California, Santa Barbara (Nov. 5,
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[13] U.S. Utility patent application Ser. No. 12/761,246, filed on
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entitled "LOW TEMPERATURE CONTINUOUS CIRCULATION REACTOR FOR THE
AQUEOUS SYNTHESIS OF ZnO FILMS, NANOSTRUCTURES, AND BULK SINGLE
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Conclusion
This concludes the description of the preferred embodiment of the
present invention. The foregoing description of one or more
embodiments of the invention has been presented for the purposes of
illustration and description. It is not intended to be exhaustive
or to limit the invention to the precise form disclosed. Many
modifications and variations are possible in light of the above
teaching. It is intended that the scope of the invention be limited
not by this detailed description, but rather by the claims appended
hereto.
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